Internal combustion engines generate nitrogen oxide (NOx) gasses as natural byproducts of the fuel combustion process. NOx gasses may be present in an engine exhaust stream in various forms, including as nitric oxide (NO), nitrogen dioxide (NO2), and/or nitrous oxide (N2O). To reduce the levels of the various NOx gasses present in vehicle tailpipe emissions, modern vehicles may be equipped with certain catalytic devices which collectively reduce these gasses to inert compounds.
Within a typical vehicle exhaust system, a suitable reductant such as ammonia or urea may be added at precisely controlled rates to the NOx-containing exhaust stream and then passed through a selective catalytic reduction (SCR) device. Catalytic action of the SCR device converts the NOx gasses into nitrogen and water. Sensors are positioned in the exhaust stream to measure NOx levels at various points of the exhaust system. Sensor measurements may be used to calculate the conversion efficiency of the SCR device and other values. The SCR device is typically replaced when the conversion efficiency drops below a threshold level.